Bacillus subtilis is a common bacterium, often referred to as hay bacillus or grass bacillus due to its prevalence in soil and plant environments. It is also found in the gastrointestinal tracts of ruminants and humans. This microorganism is frequently observed and analyzed using various microscopic techniques.
What Bacillus subtilis Looks Like
Under a standard light microscope, Bacillus subtilis appears as rod-shaped cells. These cells measure between 4 to 10 micrometers (µm) in length and 0.25 to 1.0 µm in diameter. Individual cells are often observed singly or in pairs, though they can sometimes form short chains.
A distinguishing characteristic of Bacillus subtilis is its ability to form endospores. These are dormant, refractile, oval or cylindrical bodies within the bacterial cell. Endospores are highly resistant to environmental stresses like heat, desiccation, and radiation, which contributes to the bacterium’s widespread distribution and survival in diverse habitats. These spores can be located centrally or subterminally within the sporangia, the cell that produces the spore.
How Scientists View Bacillus subtilis
Scientists employ various microscopy techniques to observe Bacillus subtilis, beginning with brightfield microscopy. Due to the small size and lack of natural contrast in bacterial cells, staining is necessary to enhance visibility under brightfield microscopes. Gram staining is a widely used differential technique that classifies bacteria based on their cell wall composition. Bacillus subtilis is classified as Gram-positive, meaning its thick peptidoglycan layer retains the crystal violet stain, appearing purple or blue under the microscope after staining.
Phase-contrast microscopy allows the observation of live, unstained bacterial cells and their internal structures. This technique converts subtle differences in light phase into brightness variations, making structures like endospores, which appear bright and refractile, readily visible without dyes. For a more detailed view of fine cellular components, electron microscopy is utilized. Transmission electron microscopy (TEM) can reveal internal structures and the multi-layered endospore coat, while scanning electron microscopy (SEM) provides high-resolution, three-dimensional images of the external cell surface, including flagella, which are hair-like appendages responsible for motility.
Why Bacillus subtilis is Important to Study
Bacillus subtilis is considered a model organism for studying Gram-positive bacteria due to its ease of cultivation and genetic manipulability. Its well-characterized genome, which contains approximately 4,100 genes, and its natural ability to take up foreign DNA (competence) make it highly amenable to genetic manipulation, including targeted gene insertions, deletions, and modifications using tools like CRISPR-Cas9. This genetic accessibility allows researchers to investigate fundamental bacterial processes such as chromosome replication, cell division, and gene expression.
The bacterium’s ability to form endospores makes it a model for studying cellular differentiation and survival under harsh environmental conditions. Beyond basic research, Bacillus subtilis has significant industrial applications. It is widely used in biotechnology for the large-scale production and secretion of enzymes, such as proteases and amylases, as well as other biochemicals and certain vitamins. Its “Generally Recognized As Safe” (GRAS) status also makes it suitable for use in food industries, for example, in the fermentation of traditional foods like natto, and as a probiotic to promote gut health.